By providing an air fuel ratio sensor to an exhaust system of an internal combustion engine, the amount of fuel to be supplied can be controlled in a feedback manner based on an air fuel ratio of an air fuel mixture obtained from the air fuel ratio sensor. However, the detected value of the air fuel ratio sensor varies in accordance with the degradation thereof. Accordingly, in order to. perform accurate feedback control, it is important to detect malfunction of the air fuel ratio sensor.
Thus, there has been known a technique in which an output value of the air fuel ratio sensor in a specific operating state is compared with a reference value thereby to detect malfunction of the air fuel ratio sensor (see, for example, a first patent document (Japanese patent application laid-open No. H10-159640) and a second patent document (Japanese patent application laid-open No. H4-8845).
However, the output value of the air fuel ratio sensor changes, other than when malfunction occurs in the air fuel sensor, due to malfunction of other devices such as, for example, malfunction in the detection of an air flow meter, malfunction in the amount of fuel injected, and so on. Thus, in case where the output value of the air fuel ratio sensor has changed, it is difficult to determine whether such a change has been caused by the malfunction of the air fuel ratio sensor or malfunction of other devices. As a result, it becomes difficult to make an accurate determination as to whether the air fuel ratio sensor is malfunction or fail. Also, this can be similarly applied to an exhaust gas temperature sensor.